Dynamics of the response of thiol monolayers to rapid heating: shadow over gold city

In this work, self-assembling thiol monolayers on gold surfaces are examined using sum frequency spectroscopy. The absorption of a laser pulse (pump pulse) increases the temperature of the metal by approximately 100 K. The absorbed energy is transferred to the molecules until they reach thermal equilibrium with the metal surface. This work focuses on the temporal change of the spectra obtained and the individual resonances in the spectra. The temporal character of the recorded changes is to be determined and conclusions drawn about the adsorption geometry of the molecules, both before and after heating the samples. This will involve estimating both the mean inclination angles of the molecules and the width of the distribution of the molecules around this mean value. Furthermore, the time courses of individual tensor elements of the second-order susceptibility (χ(2)ijk), the proportionality constants between the electric fields of the beams involved in the SFG process and the recorded intensity, are to be determined. The molecules analysed include 4-nitrothiophenol (4-NTP), 4-cyanothiophenol (4-CTP) and octadecanthiol (ODT). The molecules are adsorbed to the gold surfaces (~30 nm) through the process of self-assembly. The gold surfaces were initially deposited onto a titanium film (~13 nm) via vapour deposition, which in turn was deposited onto a glass substrate. The adsorbing molecules differ in two ways: firstly, by the tail groups under investigation ( NO2, CN, CH3), and secondly, by the spacer groups between the head and tail groups (benzyl rings vs alkyl chains). The various molecules are analysed and compared with regard to the two aforementioned molecular differences. The spectra were analysed in three different spectral ranges: 1300−1400 cm−1 (4-NTP), 2175 −2300 cm−1 (4-CTP) and 2800 −3000 cm−1 (ODT), where the stretching vibrations of the respective groups can be found. Furthermore, IR and Raman spectra are recorded for octadecanethiol and other similar molecules (11-mercaptoundecanol, 11-azidoundecanethiol) in order to ensure the correct assignment of the bands to the vibrations. This also encompasses theoretical calculations of the phase differences between different resonances, or between the same resonances in spectra recorded under different polarisation combinations. These facilitate the accurate band assignment and the selection of the optimal set of extracted fit parameters for the SFG spectra. Furthermore, thermoreflectance spectroscopy is employed to ascertain the temperature of the metal surface during pump-probe spectroscopy, which necessitates a preliminary calibration of the reflectivity changes between different temperatures. Finally, the characteristics identified for the different systems are compared at the conclusion of the thesis. A particular focus is placed on the impact of the atomic backbone of the molecules on the observed spectral changes, as well as on the influence of the spectral position of the different tail groups under investiga- tion.